1. Field of the Invention
The invention relates generally to a reference method and apparatus utilized in instruments for optical analysis. In particular, the invention relates to use of optical standards in spectroscopic systems and methods.
2. Discussion of the Prior Art
Spectrometers and References
An analyzer is a spectrometer designed for a specific purpose. Analyzers and a spectrometers have the common characteristics that incident light is delivered to a reference material, observed light is collected off of or from the reference material, and a signal is generated from the collected photons by a detector. The terms analyzer and spectrometer are utilized interchangeably herein.
A standard may be categorized as a calibrated material often with traceable or known parameters such as reflectance at a given wavelength. A standard may alternatively be referred to as a reference. A reference is utilized as a point of comparison and does not have absolutely known or universally accepted parameters. Herein, the terms standard and reference are utilized interchangeably.
Optical spectrometers may be provided in a single beam or in a double beam design. Both single beam and double beam designs commonly utilize a standard material or a reference material. Single beam spectrometers utilize a reference material in substantially the same optical path as the sample at different points in time. Double beam spectrometers utilize a reference material in a light path separate from that of the sample, but at the same point in time. In both a single beam analyzer and a double beam analyzer design, the optical reference may be air, an intensity standard, or a substance optically resembling the sample. It is common, even with the utilization of an air reference or a material with optical properties similar to a sample, to additionally utilize a reference material.
A reference material may be utilized in a number of instrument designs. First, in transmission spectrometers, references are samples utilized in designs where the incident light traverses the material and the light emits from primarily the opposite face of the material. Second, in reflectance spectrometers, mirrored surfaces reflect incident light from a spectrometer source toward a spectrometer detector without notable penetration into the reference standard. A third spectrometer design is a based upon diffuse reflectance where standards scatter incident light. A diffuse reflectance standard may be of two types, surface and body. A surface diffuse reflectance reference scatters incident light at the incident surface, such as a rough mirror. With a body diffuse reflectance reference, the incident light penetrates into the reference where the photons are scattered. Subsequently, some portion of the incident light is diffusely scattered back to the incident surface where it is emitted and coupled to the spectrometer or analyzer detector system.
A signal or spectrum collected utilizing a reference material is often referred to as a reference signal or spectrum, I0. Alternatively, a reference spectrum may be referred to as a single beam spectrum. Optical signals or spectra are utilized in a range of techniques including:                preprocessing;        processing;        calibration;        calibration transfer;        prediction; and        outlier detection.In addition, a reference standard may be utilized in determination of a spectral profile of a spectrometer, to characterize or classify a particular or group of spectrometers, or to remove instrument drift due to environmental factors such as temperature or humidity.        
A single beam spectrum may be utilized independently in analyses. However, typically a single beam spectrum is utilized in combination with a sample spectrum. A reference spectrum, I0, may be utilized in calculating transmittance, T, of a sample as in equation 1 where, I is the observed intensity or power of the sample.                     T        =                  I                      I            0                                              (        1        )            
A reference spectrum, I0, is also utilized to calculate absorbance as in equation 2 where A is absorbance and I is the observed light collected with the utilization of a sample in the optical train of the analyzer.                     A        =                                            log              10                        ⁢                                          I                0                            I                                =                                    -              log                        ⁢                                                   ⁢            T                                              (        2        )            
Often, the selection of a reference material with particular optical characteristics is important for a particular optical design. For example, in a spectrometer it is important to couple the incident light from the source to the detector via a reference material. If the interface is designed such that incident light interacting with the reference is not directed toward the detector, then the design will fail.
Description of Related Technology
A number of commercially available materials are available for use with a spectrometer for generation of a reference signal. Generally, reference materials have common properties including:                non-descript spectral signatures;        relatively featureless spectral properties;        homogeneity; and        stability.        
Practical considerations select for reference materials that are affordable, readily produced, easy to utilize, and are cleanable. In addition, a reference material should have resistance to environmental factors such as temperature, light exposure, and humidity.
A common near-IR diffuse reflectance reference material is polytetrafluoroethylene (PTFE). Pressed particles of PTFE are sold commercially under the name of LABSPHERE (North Sutton, N.H.).PTFE is a diffuse reflectance standard of approximately 99% reflectance utilized from 250 to 2500 nm. Carbon black may be impregnated into the material to form reference materials ranging from 1 to 99 percent diffuse reflective. While this type of reference is widely utilized, a PTFE body diffuse reflectance reference is expensive, requires a significant thickness, and is not readily cleaned.
G. Christensen and L. Moore, Metallization of a ceramic substrate, U.S. Pat. No. 4,526,859, (Jul. 2, 1985) describe electroplating and vapor deposition of metals onto ceramic substrates is well known in the art of surface coatings. Ceramic substrates have been selectively metallized by depositing metal at only selected portions of a metallization pattern through a customized mask. This technology is utilized for integrated circuit packaging. Coating of the entire surface would not allow creation of an integrated circuit. These materials and this methodology has not been utilized in the nonanalogous art of reference materials.
There exists, however, a need for an affordable reference material with stable optical features that may be compact in size, is resistant to environmental influences, is readily produced, and is easy to utilize.